An ultrasonic diagnostic apparatus, which is a subject information acquisition apparatus, is widely used in medical field and the like. The ultrasonic diagnostic apparatus can acquire not only morphological information reflecting an acoustic impedance distribution in a living body, but also moving information of an object such as blood flow velocity information using a Doppler technique.
To acquire the blood flow velocity information, a reflected wave from a region containing blood is used. The reflected wave from a region containing blood is mainly elastic wave (typically, ultrasonic wave) reflected or scattered from red blood cells included in the blood. The wavelength of the ultrasonic wave used by a general ultrasonic diagnostic apparatus is longer than the size of a red blood cell, so that individual red blood cells cannot be distinguished from each other. As a result, the reflected wave from a region containing blood reflects an aspect (shape, orientation, and relative position of individual red blood cells) of an aggregation of scatterers (a scatterer group) formed by a plurality of red blood cells.
To extract blood flow velocity information by using a reflected wave from a scatterer group formed by red blood cells, a technique for obtaining Doppler shift frequency of the reflected wave is often used. However, in principle, the technique for measuring Doppler shift frequency can measure only a projected component in a transmitting/receiving direction (scan line direction) of an ultrasonic beam of the blood flow velocity. In other words, to obtain an original blood flow velocity, it is necessary to perform correction considering an angle between a blood flow direction and an ultrasonic wave transmitting/receiving direction. The correction is a process of dividing a flow velocity estimated from the Doppler shift frequency by the cosine of an angle between the blood flow direction and the ultrasonic wave transmitting/receiving direction. The larger the angle is, the larger the possibility that an error increases.
Therefore, PTL 1 discloses an example that can measure a flow velocity even if the blood flow proceeds in a direction perpendicular to the ultrasonic wave transmitting/receiving direction. The method of PTL 1 acquires a blood flow velocity by calculating a cross-correlation value between two points related to a reflected/scattered waveform of an ultrasonic beam in a tomogram image and dividing a distance between the two points by a time by which the cross-correlation value between the two points reaches a peak.